1. Field of the Invention
The present invention relates to a tandem organic light emitting device. More particularly, the present invention relates to a tandem organic light emitting device using particular materials for electron injection layer, hole injection layer and metal layer connecting the electron injection layer and hole injection layer, to increase the number of the organic light emitting element contained therein without increasing the operation voltage by the theoretical fold value.
2. Description of Related Art
Organic light emitting diode (OLED) device or organic electroluminescent (EL) device usually comprises an anode, a cathode and an organic light emitting diode unit between the anode and cathode. The structure of organic light emitting diode unit includes a hole injection layer, a hole transporting layer, an organic light emitting layer, an electron transporting layer, an electron injection layer etc. Having the features of low driving voltage, high luminous intensity, wide viewing angle, and full color display, OLED is considered to have a chance to become the display device of the next generation.
Usually, an OLED is driven by current, and its current efficiency has positive correlation with the current density, but its light-emitting period has negative correlation with the current density. Therefore, how to increase the luminous efficiency without increasing the current density to prevent the decrease of the light-emitting period is an important issue in this field.
To increase the luminous efficiency and light-emitting period of OLED, the technique of series connecting known OLEDs to form tandem OLED has been developed and used. By vertically stacking several independent OLED units, one can increase the luminous efficiency by several folds under the same current density from a single power supply, but the operation voltage also has to be increased by the same folds correspondingly, which limits the application of OLED in display devices. Therefore, developing a tandem organic light emitting device is one of the key points for the commercialization of OLED.